From The Document Foundation Wiki
How can I have a sound file play throughout a presentation?
To have a sound file play throughout the presentation:
- Place it on the slide where you want it to start playing;
- In the Tasks pane, click Slide Transition;
- On the Sound dropdown list (under Modify transition), select Other Sound.... A dialog box will open.
- Browse to the desired file and open it.
The sound will continue playing until another sound file is opened.
NOTE: If the presentation is moved, the link to the sound file will be lost. See this question for more information.